The surface-hardening engineering firm Expanite has developed an advanced gaseous process for low-temperature surface hardening of stainless steel (Fig. 1). The gaseous process allows precise control for accurate tailoring of the materials’ properties and can be applied to all austenitic, martensitic, duplex and ferritic grades, imparting the material with truly unprecedented levels of wear, galling, fatigue and corrosion resistance.
Stainless steel is considered the material of choice in applications where corrosion resistance is of utmost importance. Stainless steels rely on the presence of chromium in solid solution, which allows the development and maintenance of a passive, chromium-rich oxide layer at the surface. This passive layer gives the stainless steel its “stainless” character. Unfortunately, stainless steels are relatively soft and will suffer from wear or galling when exposed to such conditions.
For many years, surface hardening of stainless steel by nitriding, carburizing or nitrocarburizing was generally considered a poor practice because treatment in the conventional temperature range of 500-950°C (932-1742°F) results in precipitation of chromium nitrides or carbides (i.e., sensitization). Although these secondary phases give rise to a hardening effect, they are also highly detrimental for corrosion properties due to the fact that chromium is removed from solid solution and thereby prevents passivation of the surface.
Furthermore, a second challenge in surface hardening of stainless steels is the impenetrability of the surface oxide, which acts as a diffusion barrier for nitrogen and carbon. This implies that in order to surface harden stainless steel, the oxide layer must first be removed prior to nitriding, carburizing or nitrocarburizing.
Since the mid-1980s, several commercial processes have been developed (mainly plasma-assisted techniques) that enabled low-temperature surface hardening of stainless steel at temperatures below 450°C (i.e., below the conventional treatment range). Incorporation of nitrogen and/or carbon at such low temperatures results in the formation of so-called expanded austenite, which is a metastable, highly supersaturated solid solution of nitrogen/carbon in austenite. Expanded austenite, or simply Expanite, is extremely hard (up to 1,800 HV) and wear resistant. It also possesses highly favorable corrosion properties since carbides or nitrides do not form.
Plasma-based treatments were first considered advantageous over gaseous processing as the removal of the passive layer – required for inward diffusion of nitrogen and/or carbon – through sputtering is an inherent part of the process. Temperature control and uniformity is limited with plasma-based techniques, however, and geometric restrictions apply. Thermodynamically controllable gaseous processing, on the other hand, enables the largest flexibility and can be straightforwardly monitored and controlled.
For many years, gaseous processing of stainless steel appeared only possible by in-situ removal of the passive layer by aggressive halogenides[2,3] or after ex-situ deposition of a metal layer that promotes the dissociation of gas components and protects the surface against (re)passivation during storage or treatment.[4,5] More recently, gaseous treatments were developed based on gas mixtures that can both remove the passive layer and provide the nitrogen/carbon to the stainless steel surface.[5,6,7]
These Expanite processes rely exclusively on gas mixtures that have this dual ability. This article describes some fundamental features of expanded austenite and technological aspects of the commercially available Expanite process.
The Nature of Expanded Austenite
Below approximately 500°C, interstitial nitrogen and carbon can diffuse over appreciable distances while the substitutional metallic elements are effectively immobile. Consequently, the development of nitrides or carbides proceeds so slowly that a nitrogen- and/or carbon-rich “case” develops that is free of chromium nitrides/carbides.
This case, or expanded austenite layer, is essentially a highly supersaturated solid solution of nitrogen and/or carbon in austenite, where the interstitial atoms group around the chromium atoms.[8,9,10] The immense hardness of expanded austenite is strictly due to an interstitial hardening effect. Typically, nitrogen contents in expanded austenite range from 20-30 atomic% N, while carbon contents are typically below 15 atomic% C. This level of supersaturation is many orders of magnitude greater than the equilibrium solubility for these interstitials in austenite at room temperature.
An example of the case produced during gaseous nitrocarburizing (ExpaniteLow-T) of AISI 316 austenitic stainless steel is shown in Fig. 2 along with the corresponding hardness and nitrogen/carbon depth profiles. The cross-sectional micrograph in Fig. 2 does not reveal a coating; rather, the vertical line running parallel to the free surface on the left represents the diffusion depth of nitrogen and carbon. Such an image is a qualitative corrosion test. The etchant selectively attacks or dissolves the “bulk” material much faster than the surface layer of expanded austenite, thereby creating a step in height between the two regions manifest as the vertical line.
Clearly, the dissolution of a colossal amount of nitrogen leads to an appreciable increase in the surface hardness along with huge (GPa-order) compressive residual stresses that arise as a consequence of an expansion of the austenite lattice within the case. The high surface hardness contributes to improved wear and galling performance, while the residual-stress profile enhances the fatigue performance.
Expanite Processes for Surface Hardening of Stainless Steel
Figure 3 shows a schematic TTT diagram of a high-nitrogen austenitic stainless steel. It is clear that, in the conventional treatment range of 500-950°C, formation of nitrides (or carbides) will take place quite rapidly. Two alternatives exist for circumventing the formation of unwanted nitrides or carbides (i.e., high temperatures, >1050°C) whereby moderate nitrogen contents can be dissolved without formation of detrimental CrN or, conversely, at low temperatures (e.g., <450°C), where a metastable, supersaturated solid solution of nitrogen and/or carbon (i.e., expanded austenite) is formed.
The ExpaniteHigh-T process is, as the name suggests, a high-temperature process where nitrogen is dissolved into the stainless steel to relatively large depths (e.g., up to 1 mm or even more). The treatment temperature is higher than the (chromium) nitride solubility temperature. All classes (austenitic, ferritic, martensitic, duplex) of stainless steel can be treated with this process. Some examples of ExpaniteHigh-T are shown in Fig. 4.
The main features of ExpaniteHigh-T on austenitic stainless steel are improved corrosion resistance and improved load-bearing capacity relative to an annealed material. The improved corrosion resistance can be quantified by the so-called pitting-resistance equivalent numbers (PREN), which rank different stainless steel alloys’ resistance to pitting. The following equation is normally applied for calculating PREN:
PREN = Cr + 3.3Mo + 16N
Clearly, the presence of nitrogen in solid solution plays a significant role for the corrosion properties. For a standard AISI 316 (18% Cr and 3% Mo), the calculated PRE number is around 28. Adding, say, 0.6 wt% N through the ExpaniteHigh-T process results in a PRE number of 37.5 – close to the PRENs for super-austenitic stainless steels (e.g., SMO grades).
Treatment of the duplex stainless steel grades (ferritic and austenitic) results in a nitrogen-stabilized austenitic case with a duplex core (Fig. 4b). The nitrogen-enriched austenite case offers improved corrosion and mechanical properties compared to non-treated duplex stainless steel. In particular, the erosion-corrosion resistance is improved (e.g., cavitation erosion).
The martensitic and ferritic grades show the largest response to ExpaniteHigh-T treatment with respect to hardness. In the martensitic grades, a case of nitrogen-enriched martensite is formed while the core consists of carbon martensite (Fig. 4a). The ferritic steels are in this sense more straightforward because the core of the material does not experience a martensitic transformation during quenching. When these steels are subjected to ExpaniteHigh-T, nitrogen will stabilize an austenitic surface layer that upon quenching forms nitrogen-enriched martensite. The core remains ferritic throughout the process (Fig. 4c). The end result is a nitrogen martensitic case on a ferritic core. Nitrogen martensite offers a very high surface hardness, up to 700-800 HV, depending on the treatment and alloy, as well as significantly improved corrosion resistance.
The other alternative for surface hardening of stainless steel without impairing the corrosion resistance is to apply low temperatures whereby nitrogen and carbon remain in supersaturated solid solution. ExpaniteLow-T is a nitrocarburizing process that results in an outermost surface layer of nitrogen-expanded austenite and an inner layer of carbon-expanded austenite. This low-temperature process presents the best of both worlds – high surface hardness and chemical resistance from nitrogen – and bridges surface and bulk properties by the underlying layer of carbon-expanded austenite. The ExpaniteLow-T process can be applied to all stainless steel varieties and several other materials systems, such as the Co- and Ni-based alloys.
The amount of carbon and nitrogen in the expanded-austenite case, as well as the case depth, can be tailored with the gaseous approach for specific properties. The properties of an expanded-austenite surface include highly improved wear resistance, galling resistance, fatigue resistance (due to the presence of high compressive stresses), and dramatically improved pitting and crevice corrosion resistance (from the high interstitial concentration of nitrogen).
The improvement against pitting corrosion is observed in the anodic polarization curves given in Fig. 5. The untreated AISI 316 reference has a distinct pitting potential at around 400 mV whereby pitting is initiated. The ExpaniteLow-T sample does not experience pitting, even at high potentials (where it exhibits trans-passivity). Furthermore, the free corrosion potential of the treated samples is higher (more noble), and the current in the passive region (-100 to +500 mV) is lower. Similar improvements are obtained through other corrosion-testing methods (e.g., salt-spray testing, submerging tests in aggressive media, etc.).
For demanding applications where very high corrosion resistance is required together with good load-bearing capacity, the ExpaniteLow-T surface hardening process can be preceded by an ExpaniteHigh-T treatment. Essentially, this is a combination of the two processes described above. Figure 6 shows examples of austenitic AISI 316 and duplex SAF 2205 treated with SuperExpanite. This process provides the best solution for general wear and corrosion resistance.
Surface hardening of stainless steel can be achieved via specialized high- and low-temperature gaseous thermochemical processes that transform the surface into N/C-expanded austenite. Gaseous processing with the ExpaniteHigh-T, ExpaniteLow-T and SuperExpanite processes provides a high degree of tailorability of the hardened case with unprecedented levels of tribological and electrochemical performance. Nearly all stainless steels and similar alloy systems can be surface hardened with the Expanite process.
For more information: Contact Christian Dalton, Ph.D., N. America Sales Manager & Process Specialist, Expanite Inc.; 8250 Boyle Parkway, Twinsburg, OH 44087; tel: 330-998-7285; e-mail: firstname.lastname@example.org; corporate tel: +45 2819 6443; e-mail: email@example.com; web: www.expanite.com
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